There have been many suggestions of catalysts which are used in manufacturing methacrylic acid by a vapor phase catalytic oxidation reaction of methacrolein (this vapor phase catalytic oxidation reaction will be referred to merely as “oxidation reaction,” unless otherwise specified hereinafter). These suggestions are mainly related to elements constituting the catalysts and a ratio of the elements.
The oxidation reaction causes heat accumulation in a catalyst bed because this reaction is an exothermic reaction. Locally abnormal high temperature zones resulting from excessive heat accumulation are known as hot spots, in which yields are reduced by the excessive oxidation reaction. Thus, when conducting the oxidation reaction on an industrial scale, occurrence of the hot spots is a significant problem, and particularly in the case where a concentration of methacrolein in a raw material gas is increased in order to increase productivity, the hot spots tend to easily occur, so that the reaction conditions are required to be largely restricted under the present circumstances.
Therefore, it is very important to control the temperature at the hot spot section for producing methacrylic acid in high yield on an industrial scale. In addition, especially in case of using a molybdenum containing solid oxidation catalyst, it is important to prevent the occurrence of the hot spots because the molybdenum moiety is easy to be sublimated.
Several suggestions have been made as for a method for controlling the temperature at the hot spot section. For example, Japanese Patent Application Laid-Open No. 4-210937 discloses a method in which a plurality of catalysts having different activities are filled in such a manner that their activities become gradually higher from an inlet side for a raw material gas toward an outlet side, then the raw material gas containing methaclorein and oxygen is flown through this catalyst bed. Japanese Patent Application Laid-Open No. 8-92154 discloses a method in which, when acrolein is subjected to vapor phase oxidation using a multi-tube type fixed bed reactor provided with a heat medium bath to produce acrylic acid, a flow of the heat medium is controlled so that the temperature of the heat medium bath is raised by 2 to 10° C. between the inlet section and the outlet section.
However, these methods are only for the purpose of decreasing the temperature of the hot spot section, and are the methods which merely reduce a temperature difference (ΔT) between the heat medium bath and the catalyst bed in one hot spot section to some extent. That is, according to these methods, heat generation caused by oxidation reaction has not been actively controlled within the catalyst bed, so that, for example when a methacrolein concentration is further increased in order to increase productivity, heat generation at the hot spot section becomes remarkable. Hence, the reaction conditions are still required to be largely restricted. That is, under the present circumstances, the yield of methacrylic acid which is industrially acceptable has not been obtained when a concentration of methacrolein is increased to a level which is industrially acceptable.